Nitrile rubbers, also abbreviated to “NBR”, are rubbers which are co- or terpolymers of at least one α,β-unsaturated nitrile monomer, at least one conjugated diene and optionally one or more additional copolymerizable monomers. Nitrile rubbers of this type and processes for producing such nitrile rubbers are known, see, for example, Ullmann's Encyclopedia of Industrial Chemistry, VCH Verlagsgesellschaft, Weinheim, 1993, p. 255-261.
NBR is typically produced by emulsion polymerization to initially obtain an NBR latex. The NBR solid is isolated from this latex by coagulation, usually using salts or acids. The emulsion polymerization is typically carried out using molecular weight regulators. Commonly used molecular weight regulators are based on mercaptans. The use of dodecyl mercaptans is of particular importance for molecular weight regulation of emulsion rubbers based on monomers such as styrene, butadiene, acrylonitrile, (meth)acrylic acid, fumaric acid, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, chloroprene and others.
U.S. Pat. No. 2,434,536 describes that synthetic rubbers based on diolefins, for example butadiene and optionally additional copolymerizable monomers, for example styrene, α-methylstyrene, vinylnaphthalene, acrylonitrile, methacrylonitrile, methyl methacrylate, ethyl fumarate or methyl vinyl ketone, are produced by emulsion polymerization in the presence of aliphatic mercaptans as molecular weight regulators. It is disclosed that these mercaptans comprise at least 7 and preferably 10 or more carbon atoms. It is preferable to use aliphatic mercaptans having a mean molecular weight of from 188 to 230 which comprise at least 50% dodecyl mercaptan and which comprise the balance to 100% in the form of mercaptans having 10 to 16 carbon atoms.
Ullmanns Enzyklopädie der technischen Chemie [Ullmann's encyclopedia of industrial chemistry] 4th edition, volume 13, page 611-612 describes in general terms that the molecular weight of nitrile-butadiene rubbers can be controlled by using alkyl mercaptans, di- and polysulfides or xanthogen disulfides. tert-dodecyl mercaptan and diisopropylxanthogen disulfide are named as the regulators mainly used. Tertiary dodecyl mercaptans (also abbreviated to “TDM” or “TDDM”) are also often used in industrial practice. The TDM commercially available from Chevron Phillips is known, for example, and generally consists of a large mixture of a very wide range of isomers.
Whether, or to what extent, volatile substances outgas from vulcanizates produced on the basis of nitrile rubber is relevant for a very wide range of applications, for example floor coverings of nitrile rubber. Applicant studies showed that nitrile rubbers produced using TDM comprise a high proportion of sulphur compounds and, furthermore, non-sulphur-containing impurities in the TDM in VOC tests (carried out by means of TDS GC-MS analyses according to VDA recommendation 278 September 2002). Depending on the mode and purpose of the application these can outgas in practical use and can lead to noticeable, unpleasant odour nuisances that may become unacceptable.
While there is extensive literature in existence about the influence of the salts that can be used for latex coagulation on the properties of the NBR obtained, there are no indications or studies concerning the influence of the molecular weight regulators on the volatile constituents contents in nitrile rubbers.
DD 154 702 discloses a process for free-radical copolymerization of butadiene and acrylonitrile in emulsion, which is controlled via a specific, advantageously computer-aided metered addition programme for the monomers and the molecular weight regulator, for example tert-dodecyl mercaptan, and in which the latices obtained are worked up by coagulation in an acidic medium to give the solid rubber. A significant advantage of the process is stated to be that, due to the use of acids in the coagulation, the resin soaps and/or fatty acid soaps used as emulsifiers remain in the rubber, i.e., are not washed out as in other processes. This is claimed not just to have the advantage of good properties of the NBR but particularly also to improve the economics of the process and to avoid wastewater pollution by washed-out emulsifier. It is stated that the butadiene-acrylonitrile copolymers comprising 10-30% by weight of acrylonitrile feature good elasticity properties and low-temperature properties combined with elevated swell resistance and advantageous processability. Measures making it possible to influence the VOC values of the nitrile rubber or the profile of properties of the vulcanized NBR cannot be inferred from the teaching of this patent.
EP-A-0 692 496, EP-A-0 779 301 and EP-A-0 779 300 each describe nitrile rubbers based on an unsaturated nitrile and a conjugated diene and said nitrile rubbers each comprise 10-60% by weight of unsaturated nitrile, have a Mooney viscosity in the range of 15-150 or, according to EP-A-0 692 496, of 15-65 and comprise at least 0.03 mol of a C12-C16-alkylthio group per 100 mol of monomer units, this alkylthio group comprising at least three tertiary carbon atoms and a sulphur atom bonded directly to at least one of the tertiary carbon atoms. Each of the nitrile rubbers is produced in the presence of a C12-C16-alkyl thiol of appropriate structure as molecular weight regulator which functions as a “chain transfer agent” and is thus incorporated into the polymer chains as an end group.
It is stated that the nitrile rubbers according to EP-A-0 779 300 have an unsaturated nitrile composition distribution breadth “ΔAN” (AN stands for acrylonitrile) in the copolymer in the range of from 3 to 20. The process for their production differs from that of EP-A-0692496 in that only 30-80% by weight of the total monomer amount is employed on commencement of the polymerization and the remaining monomer amount is only metered in at a conversion of the polymerization of 20-70% by weight.
It is stated that the nitrile rubbers according to EP-A-0 779 301 comprise 3-20% by weight of a fraction having a low molecular weight having a number-average molecular weight Mn of less than 35 000. The process for the production of said rubbers differs from that of EP-A-0 692 496 in that only 10-95% by weight of the alkylhiol are mixed into the monomer mixture prior to the polymerization and the remaining amount of the alkylthiol is metered in only once a polymerization conversion of 20-70% by weight has been attained.
According to EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301, it is in each case essential for the production of the nitrile rubbers to use alkylthiols in the form of the compounds 2,2,4,6,6-pentamethylheptane-4-thiol and 2,2,4,6,6,8,8-heptamethylnonane-4-thiol as molecular weight regulator. In this connection it is pointed out that nitrile rubbers having poorer properties are obtained when the conventional, known tert-dodecyl mercaptan is used as regulator.

It is asserted that the nitrile rubbers produced in EP-A-0 692 496, EP-A-0 779 300 and EP-A-0 779 301 have an advantageous profile of properties and that they facilitate good processibility of the rubber mixtures and low mould soiling on processing. The vulcanizates obtained are said to have a good combination of low-temperature stability and oil resistance and good mechanical properties. Emphasis is furthermore placed on the high productivity of the production process due to high polymerization conversions of more than 75%, preferably more than 80%, the high vulcanization rate in the vulcanization with sulphur or peroxides, in particular for NBR-types for injection moulding, a short scorch time of the nitrile rubbers and a high crosslinking density. Neither EP-A-0 692 496 nor EP-A-0779 300 nor EP-A-0779 301 give indications as to the influence, if any, of the molecular weight regulators used on the properties of the NBR and the emission characteristics thereof.
WO-A-2001/094432 discloses specific branched nitrile rubbers which, at a content of bound unsaturated nitrile of from 15% to 50% by weight and a Mooney viscosity (ML 1+4 at 100° C.) in the range of from 15 to 150 Mooney units, exhibit chain branching in the range of from 0° to 20° (determined by what is known as the ΔδB value) and a solubility measured in methyl ethyl ketone at 20° C. of ≧85% by weight. These nitrile rubbers are provided by effecting the polymerization using a molecular weight regulator, wherein the regulator is not added to the polymerization mixture in one charge, i.e., at once, but rather in at least two stages, preferably three or more stages, though continuous addition over the entire polymerization time is also possible. According to WO-A-2001/094432, various chain regulators can be used and these are mentioned in EP 0 799 300 B1 on page 3, lines 51-58 and page 4, paragraph 3. Preference is given to alkyl thiols, such as 2,4,4-trimethylpentane-2-thiol, 2,2′,4,6,6′-pentamethylheptane-4-thiol, 2,2′,4,6,6′,8,8′-heptamethyl-nonane-4-thiol and mixtures thereof.
WO-A-2008/142042, WO-A-2008/142035 and WO-A-2008/142039 each disclose processes used to produce specific NBR nitrile rubbers having particular ion contents and ion indices and specific properties associated therewith both in the rubber and in the corresponding vulcanizates. Specific dodecyl mercaptans, for example that described in WO-A-2008/142037, can be used for molecular weight regulation in these processes. Accordingly, fragments of the regulator substances used are found in the polymer chains.
The production of butadiene/acrylonitrile copolymers in the presence of various primary, secondary and tertiary mercaptans was studied in J. Appl. Polym. Sci. 1968, Vol. 12, 1075-1095. A particular focus was the performance of the mercaptans in terms of their ability to control the molecular weight and the Mooney viscosity of the polymers. The poorest results were attained with n-alkyl mercaptans. By contrast, secondary mercaptans such as 2-nonyl mercaptan, 2-decyl mercaptan and mixtures proved to be more efficient regulators at low temperatures (5° C.). Tertiary C7 to C13 mercaptans showed the best results. Optimal transfer constants for a 70/30 butadiene/acetonitrile copolymer and an 80/20 butadiene/acetonitrile copolymer were obtained in a polymerization at 5° C. When using tert-nonyl mercaptan the polymerization was carried out up to a conversion of no more than 59%. No indication whatsoever is given as to the influence, if any, of the regulator substances analysed on the volatile substances content in the butadiene/acrylonitrile rubbers produced and the extent to which the other properties of the rubbers and vulcanizates based thereon are influenced.
In summary it can be stated that, to date, there are still no known measures by which mercaptans can be used as molecular weight regulators to obtain nitrile rubbers having a distinctly reduced volatile substances content and simultaneously a profile of properties which remains unchanged and good, particularly in terms of the vulcanizate properties.